Nowadays, optical disks can be found almost everywhere. Whether they are used to hold music or data, they have become the standard medium for distributing large quantities of information in a reliable package. A common optical disk is a 120 mm diameter disk of polycarbonate plastic, having a single spiral track of data, which is constructed by the “pits” and “lands”, circling from the inside of the disc to the outside. If the distance between the designated data point and center of the optical disk is r, and the rotating speed of the spindle motor is ω, the relative speed of the data point V with respect to the optical head can be expressed asV=r·ω. From, it is understood that one of the three variables (V, r, ω) has to be constant. Since the radius r varies with the position of the data point, only V or ω can be constant. Therefore, the spindle motor can generally rotate in two modes: Constant Linear Velocity (CLV) and Constant Angular Velocity (CAV).
If the servo system keeps V constant, the rotation mode of the spindle motor is called CLV. The advantages of CLV are that the data transfer rate is kept fixed, and that the phase lock loop (PLL) can maintain the correct data readout only by locking a fixed frequency. However, the rotating speed of the spindle motor has to change synchronously with respect to the position of the optical head. If the rotating speed keeps increasing to some extent, it shall be put into consideration that whether the spindle motor can achieve the predetermined high speed when the optical head is in inner tracks.
If the servo system keeps ω constant, the rotation mode is called CAV. It is easier to control the spindle motor in CAV mode since the spindle motor rotates with a fixed angular velocity, which is just different from CLV. However, the data transfer rate in outer tracks is higher than that in inner tracks. Therefore, the PLL has to follow the data readout to modify the fundamental frequency that should be locked when in CAV mode. Meanwhile, attention should be paid to the stability of data readout.
By virtue of this, an optical drive can either select the CLV mode or the CAV mode for controlling the rotation speed of the spindle motor while the optical pick-up head (PUH) is reading/writing an optical disk, and the same time, both modes control the laser power outputted from the PUH basing on a value of constant linear velocity (which is referred as CLV value hereinafter). In another word, the CLV mode is based on the number of revolution for controlling the laser power outputted from the PUH of an optical drive, and the CAV mode is based on the CLV values detected in the optical disk.
Please refer to FIG. 1, which is a circuitry of a DVD burner adopting CAV mode for laser power control. As seen in FIG. 1, the DVD burner 100 uses a spindle control unit of CAV mode 106 for controlling a spindle motor 104, and when an optical pickup head 105 is used for reading/writing an optical disc 103, it will output a number of signals to a synchronization signal decoder 107 for enabling the same to output a number of synchronization signals 151 to a CLV value detector 101 such that the CLV value detector 101 is enabled to calculate a CLV value 153 basing on the received synchronization signals 151 and output the calculated CLV value 153 to a laser power control 102 for enabling the same to issue a certain control signals to an automatic power control 108 where the control signal is adjusted and then fed to a laser drive for controlling the laser power outputted from the optical pickup head 105.
Refer to FIG. 2, which is a profile showing relationship between CLV values of FIG. 1 and laser powers outputted from an optical pickup head. According to the profile shown in FIG. 2, the laser power control 102 can direct the optical pickup head 105 to discharge an intended laser power by interpolation based on the CLV value detected by the laser power control 102. For example, if the CLV value 153 detected by the laser power control 102 is a value addressed as CLVx ranged between CLV1 and CLV2, the laser power Px can be acquired by interpolating the profile of FIG. 2, wherein, the CLV1 is the CLV value 153 detected while the optical pickup head 105 is reading/writing an inner track of the disc 103 in CAV mode, and the CLV2 is the CLV value 153 detected while the optical pickup head 105 is reading/writing an outer track of the disc 103 in CAV mode.
Nevertheless, the track of a DVD disc is not as wide as that of a VCD disc, that is, the track of DVD disc is narrower (i.e. about 0.74 mm) and the track of VCD disc is wider (i.e. about 1.6 mm). By virtue of this, since the conventional method of using the profile of FIG. 2 as a base for controlling the laser power discharged from the optical pickup head of a DVD burner lacks the consideration of track width, it is easy to damage the track of DVD disc during a burning process.
In view of the aforesaid description, the present invention provides a device and method of polynomial power control for optical drives, capable of adapting to the narrower track of DVD disc by controlling the optical pickup head to discharge a designated intensity of laser power during a burning process.